run: executed cmd is not terminated when baur process dies

[fho]

How to reproduce:

• run a long running process (sleep 10m) as baur task,
• send SIGTERM to the baur process

result:

• the sleep process is still running

expected result:

• the executed task terminates when the baur process terminates

Fixed:

• [x] Linux & FreeBSD: https://github.com/simplesurance/baur/pull/404
• [ ] windows: https://github.com/golang/go/issues/17608 support required?
• [ ] mac os: ?